Make Gradient between two gradient spheres
You can deduce the scales, and therefore the radii, of your Empties, by dividing the shaded object's Object-Space length to the shading-point, by the same length, in the Empty's spaces. (The Empties are set to display at 1 unit, and are at the object's origin in XY):

..The range between the radii can be mapped to (1-&gt;0), and clamped, using a Map Range node.
Here illustrated by putting the range through a color-ramp, and scaling the Empties:

.. Sorry, I can't get the Empties to show up very well in the .gif, but they are at the bounds of the gradation.